fix: temporarily allow forked repos to run PR workflows (#3503)

* fix(zcash-params): Do not update parameters image on PR

We should not update a direct dependency of our Docker image to be writeable by a PR from anywhere, a local branch or a fork branch, before that change has been approved by a human and merged to #main

Co-authored-by: Deirdre Connolly <durumcrustulum@gmail.com>

.github/workflows/zcash-params.yml

@@ -2,7 +2,9 @@ name: zcash-params
 
 on:
   workflow_dispatch:
-  pull_request:
+  push:
+    branches:
+      - 'main'
     path:
       - 'zebra-consensus/src/primitives/groth16/params.rs'
       - 'zebra-consensus/src/chain.rs'